The present invention relates generally to transmitters for transmitting angle modulated signals, and, more particularly, to a digital premodulation filter for conditioning a serial bit stream prior to frequency (or phase) modulation of a RF carrier signal.
For radio communication constant-envelope frequency modulation signals are preferable due to existing system constraints in power economy and the high efficiency amplification available with non-linear power amplifiers. A disadvantage of frequency modulation (FM) is however that the spectrum is rather wide. A solution is to use spectrally efficient modulation to maximize the bandwidth efficiency, measured in bits/second/Hz. A method for achieving spectrally efficient modulation is to shape the data at the input of the frequency modulator by means of a premodulation filter. Such a premodulation filter is described in the article "Tamed Frequency Modulation, A Novel Method to Achieve Spectrum Economy in Digital Transmission" by F. deJager and C. B. Dekker, published in "IEEE Transactions on Communications" Vol. Com. 26 No. 5, May, 1978. The modulation described in this article relates to what is commonly referred to as tamed frequency modulation (TFM).
U.S. Pat. No. 4,477,916 entitled "Transmitter for Angle--Modulated Signals" issued to Kah-Seng Chung on Oct. 16, 1984, discloses an FM transmitter having a premodulation filter to shape an input data signal in a predetermined manner. The premodulation filter comprises a Gaussian low-pass filter and a correction filter. The latter filter comprises a cascade of 2n delay sections, each having a time delay of T seconds. The output of the premodulation filter is connected to the signal input of a frequency modulation circuit to provided the modulated output as desired.
U.S. Pat. No. 4,531,221 entitled "Premodulation Filter For Generating A generalized Tamed Frequency Modulated Signal", issued to Kah-Seng Chung and Leo E. Zegers on July 23, 1985 discloses a transmitter having a premodulation filter arranged to provide a substantially three-level signal at the sampling instants t=(2m-1)T/2, wherein T is the symbol duration of the data signals and m is an integer. The premodulation filter comprises the series arrangement of a non-recursive second order digital filter with three weighing factors, two of which are equal to each other, and a low pass filter realized by means of a "raised cosine" filter which satisfies the first Nyquist criterion. The mutually unequal weighting factors have the values A and B, the value of the weighting factor A and, respectively of the non-recursive second order filter being less than one-fourth and larger than one-half, respectively, 2A+B being equal to one and the roll off coefficient of the "raised cosine" filter being unequal to zero.
The above-mentioned prior art transmitters utilize premodulation filter circuits comprising analog filters or a combination of digital and analog filters to provide carrier modulation dedicated to a single modulation type; TFM, for example.